With the advent of internet-connected devices and the digital health industry, health and wellness monitoring has become an area of growing focus. Monitoring vital signs such as heart rate, ballistocardiogram signals, and breathing rate is desirable both inside and outside healthcare facilities. Within healthcare settings, vital sign tracking can be essential for: ensuring patient safety when a healthcare provider is not present at a bedside, diagnosing medical conditions, monitoring a patient's progress, and planning a patient's care. Outside of healthcare settings, tracking vital signs and posture enables individuals to quantify and conceptualize their health status, thereby helping individuals remain mindful of their health and wellness needs, visualize progress, and maintain the motivation needed to achieve health and fitness goals.
Current vital sign trackers in the consumer market are fairly intrusive, for example, current heart rate monitors often require an individual to strap the monitor around the individual's chest. Many vital sign trackers include just one type of sensor configured to detect one type of vital sign, such as, for example, heart rate. Additionally, many vital sign monitors in the consumer market are not very accurate. In the healthcare setting, much more accurate devices are available, but they are often very large devices positionable at a patient's bedside, requiring a connection to an electrical outlet and leads attached to the patient. Attachment to these bedside devices can cause anxiety in patients, and the devices are expensive, not portable, and prone to electromagnetic interference (EMI).
Optical fiber sensors have gained increased attention in the research setting as an alternative to existing vital sign monitors. Optical fiber sensors are chemically inert and resistant to EMI. Moreover, they can be portable and integrated into fixtures, such as mattress pads and cushions. Fixture-integrated devices have numerous advantages over bedside appliances and wearable instruments. For example, fixture-integrated devices allow for a reduction in loose connecting wires or wireless data transmitters between sensors, electronics, and power supplies. This reduction may lead to increased reliability, data quality, and security.
However, optical fiber sensors developed to date have not proven to be suitable alternatives to conventional monitoring systems. For example, in “Optical Fibre Sensors Embedded into Medical Textiles for Healthcare Monitoring,” IEEE Sensor J. 8 (7), 1215-1222, 2008, Grillet et al. proposed integrating a single mode macro-bending fiber sensor into a belt to measure respiratory rate. A macro-bending sensor typically experiences significant light loss due to macroscopic deviations in the fiber's axis from a straight line, resulting in low sensitivity. Such a sensor would be unlikely to detect the subtle movements of the chest wall needed to accurately measure heart rate or ballistocardiogram signals.
In an effort to improve sensitivity, others have proposed alternative approaches for fiber optic sensors. For example, in U.S. Pat. No. 6,498,652, Varshneya et al. disclosed a fiber optic monitor that utilizes optical phase interferometry to monitor a patient's vital signs. Optical phase interferometry has several limitations, for example, it requires an expensive phase modulator and coherent optical sources, which adds significant cost and complexity and makes it impractical for widespread commercial adoption. Other proposed designs have struggled to balance sensitivity, accuracy, and cost. Thus, there is a need for new and useful optical fiber sensors.